> Dear All: > What does it mean when I get negative values under Anomalous Corr > column after running XDS? I set the Friedel Law=False even though I > suspect that my signal is very very weak. > Thanks > Subbu
Hi Subbu,it means that in resolution shells with negative Anomalous Corr the anomalous signal is too weak to be useful. If you have no anomalous signal, the Anom CC is not necessarily zero, but rather (resulting from noise) is a number drawn from a normal distribution centered around zero - which means it may be negative or positive, with low absolute value (< 0.5). If it is negative throughout several resolution shells, I'd suspect radiation damage; negative values may result in certain spacegroups and data collection geometries when radiation damage is dominating the differences between symmetry/Friedel-related observations.
But make sure you take the resolution correctly into account: it doesn't matter much if low or negative Anom CC occur at high resolution. Rather, that's to be expected for non-lyozyme/insulin/thaumatin/xxx crystals.
Substructures can be solved with 6A data - so if you have a high Anom CC (say, > 0.75) in the first resolution shell(s) then this is reason for hope.
In your case I'd probably set FRIEDEL's_LAW=TRUE for CORRECT (because it helps in scaling, at least if multiplicity is low) and switch to FRIEDEL's_LAW=FALSE for xdsconv.
HTH, Kay -- Kay Diederichs http://strucbio.biologie.uni-konstanz.de email: kay.diederi...@uni-konstanz.de Tel +49 7531 88 4049 Fax 3183 Fachbereich Biologie, Universität Konstanz, Box M647, D-78457 Konstanz This e-mail is digitally signed. If your e-mail client does not have the necessary capabilities, just ignore the attached signature "smime.p7s".
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